Inductance apparatus



No v. 2, 1943.

N. BOTSFORD INDUCTANCE APPARATUS Filed June 28, 1941 LEAKAGE MUCH/WE LEAKAG! J INDUCDINCE LEAKAGE LEA/(46C INDUCTZNCE TRANSMISSION L INE' MGM'TIC CORE INVENTOR N. BOTSFORD AT TORNEV Patented Nov. 2, 1943 r Nelson Botsi'ord,

Rutherford, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 28, 1941, Serial No. 400,339

Claims.

This invention relates to inductance apparatus, and more particularly to inductance apparatus for transmitting signaling waves extending over, a relatively broad band of frequencies with substantially uniform attenuation and relatively low reflection. v

Carrier current transmission systems are being presently designed with higher frequencies A further object is to simplify the manufacture of: inductance apparatus.

' A further object is to compensate for the leakageinductance of inductance apparatus in so far as such inductance affects reflection and transmission.

to provide a larger number of discrete carrier channels. This means that certain inductance apparatus, such as repeating coils and:transformers, is 'also being designed to transmit a wider band of frequencies. such inductance apparatus, one factor involves controlling the reflection characteristic. This characteristic is among others a function of leakage inductance, which inductance isthe effective pling of the windings of the inductance apparatus. leakage inductance may be partially annulled by the effective shunt capacitance of the inductance apparatus. However, in line repeating coils and transformers where impedance levels are relative low, the amount of leakage inductance may be found to be intolerable. In this connection it has also been found that the amount of leakage inductance is further increased because of design, features which tend to minimize modulation effects in the inductance apparatus;

Heretofore, the amount of leakage inductance, not annulled by the effective capacitance of the inductance apparatus, has been partially neutralized by connecting individual fixed capacitors in shunt of the input and output terminals of the inductance apparatus. These capacitors served to function together with the leakage inductance as a low-pass-filter having a characteristic impedance of the same order of magnitude as the winding terminations of'the inductance apparatus so as to transmit therethrough with tolerable reflection and attenuation signaling waves extending over a certain range of frequencies.

controlling the reflection characteristic of inductance apparatus. t

The main object of the invention is to provide inexpensive, high quality, inductance apparatus.

In the design of series inductance caused by an imperfect coufor such compensation on the bases of individual Another object is to provide in a system proper terminations for apparatus which functions most effectively, only when terminated in the impedance out of and into which such apparatus is designed to Operate. Another'object is to minimize between adjacent transmission lines crosstalk caused by reflected waves thereon.

Onetype of well-known inductance apparatus,

such as a repeating, coil, comprises a pair of windings wound on a magnetic core and between which a certain amount of leakage inductance exists, that is, inductance due to magnetic flux which is not mutual to the pairof windings. One winding of this'pair may be applied, for example, to central ofilce equipment while the other winding maybe connected to a carrier currentline transmitting signaling waves extending overa certain range of frequencies. As an uncertain amount of leakage inductance may be introduced in the inductance apparatus due to normal manufacturing variations, it has been found diificult to compensate for the latter amount of leakage inductance without providing inductance apparatus. Unless due care is observed to compensate for the leakage inductance of the inductance apparatus, it may happen that I an intolerable impedance mismatch occurs between the inductance apparatus and the carrier current line or between the inductance apparatus and the central office equipment. Such impedance mismatch, depending on the amount, may

In some cases, adjustable capac-jitors may be required to obtain a finer control increase the reflection characteristic of the inductance apparatus to such extent that transmission in the carrier current system is deleteriously afiected. I v

In a specific embodiment of this invention, an impedance, network comprising a tertiary winding and a capacitor in shunt thereof is coupled to the pair of 'windingsof the inductance apparatus. The magnitude of the capacity of the capacitor is initially selected at a value suitable withrespect to capacitance, physical dimensions and commercial availability, and thereafter the number of turns of the tertiary winding adjusted to neutralize substantially whatever amount of leakage inductance is then present in the inductance apparatus. Thus, the impedance of the inductance apparatus looking 'into the other winding with the one winding applied to the central office equipment, is substantially matched to the impedance of the carrier current line;' and the impedance of the inductance apparatus, looking into the one winding with the other winding connected to the carrier current line, is substantially matched to the impedance of the central office equipment.

A feature of the invention is that it may be expeditiously utilized in various types of inductance apparatus, such as repeating coils, transformers and hybrid coils.

The invention will be readily understood from the following description taken together with the accompanying drawing in which:

Fig. 1 is a schematic circuit showing a specific embodiment of the invention applied to a repeating coil; and

Figs. 2 and 3 are schematic circuits illustrating the specific embodiment of the invention embodied in hybrid coils.

Fig. 1 shows a repeating coillll comprising a winding II whose terminals 9, 9 are applied to a load I2, which, for example, may comprise central office equipment, and a winding I3 whose terminals I4, I4 are applied to a source 8 of alternating current waves and a resistor I, both of which, for example, may simulate a suitable carrier current transmission line 6. The windings II and I3 are wound on a magnetic core I5, and also may have interposed therebetween a shield IS. The coil I may also be enclosed in a casing I1 which together with the shield I6 is connected to a ground I8. An amount of leakage inductance will be present between the windings II and I3, and may be represented for the purpose" of illustration as inductance I9 in series with one terminal I4.

The coil I0 is theoretically designed to have a substantially fiat impedance-frequency charac-' teristic that conforms substantially with substantially flat impedance-frequency characteristics of the central office equipment and the carrier current line to transmit all signaling waves extending over a certain range of frequencies with substantially uniform attenuation. This means that, with the winding II terminated in the load I2, the impedance termination of the coil Ill, looking into the winding I3 ought to match substantially the impedance termination of the carrier current line 6; and with the winding I3 terminated in the carrier current line 6, the impedance termination of the coil I0 looking into the winding I I ought to match substantially the impedance termination of the load I2.

However, leakage inductance present in the coil I U, as previously mentioned, will have the effect of upsetting such impedance match, that is, causing a mismatch between the impedance looking into the windings II and I3 and the impedance of the respective carrier current line 6' and the load i2. Such impedance mismatch, controlled by the magnitude of the leakage inductance, tends to vary the reflection characteristic of the coil ID, thereby exerting a deleterious effect on transmission in the entire carrier cur-- rent system of Fig. 1 from the standpoint of both efiiciency and fidelity, as will be hereinafter pointed out. Control of the reflection characteristic in inductance apparatus is therefore an important factor in the over-all design of carrier current systems.

Thus, in order to operate filters and hybrid coils embodied in central office equipment sub' stantially in accordance with their design requirements, it is imperative that such equipment be connected to the impedance out of and into which it is designed to operate. Furthermore, in order to minimize the effects of reflection and crosstalk on carrier current transmission lines,

ductance apparatus is terminated in the line, the

central office winding is terminated in an impedance which is substantially equal to the impedance of the central oifice equipment. Therefore, the impedance looking into either the line or central office winding of the inductance apparatus I0 must, to a tolerable degree, hold the match to the respective line B or central office equipment I2 over the entire frequency range of the signaling waves being transmitted in the system.

In accordance with this invention, the impedance looking into winding I3, when the winding II is applied to the load I2, is adjusted for signaling waves extending over a certain range of frequenciesso as to be substantially equal to the impedance of the carrier current line 6 connected thereto and transmitting said signaling waves extending over said certain range of frequencies with substantially uniform attenuation; and the impedance looking into the winding II,. when the winding I3 is connected to the carrier current I line 6, is adjusted so as to be substantially equal to the impedance of the load I2 for the same range of signalingfrequencies.

Referring to Fig. l, the foregoing is accomplished by a tertiary winding 20 coupled to the windings II and I3 and across which is connected a capacitor 2I. The capacity of the capacitor 2I is initially selected at a value suitable with respect to capacitance, physical proportions and commercial availability, and thereafter the number of turns of the tertiary winding 20 is adjusted to neutralize substantially the leakage inductance present between the windings II and I3, that is, the leakage inductance I9, thereby providing substantially fiat effective resistive and minimum reactive-frequency characteristics for the coil I0 either looking into the winding I3 from the carrier current line 6, or looking into the winding II from the load I2, when the other of these two windings is properly terminated in the manner previously mentioned. Hence, the attenuation-frequency characteristic of the coil I0 j tends to conform substantially with the attenuation-frequency characteristics of the respective carrier current line 6 and load I2.

Maintaining the impedance match between the terminated windings II and I3 in the manner pointed out above serves (1) to control the effecance enables a control of the reflection charac-- teristic of the coil ill so as to promote both high efiiciency and high quality in the operation of the entire carrier current system of Fig. 1.

In connection with Fig. 1, it was found that, for a specific embodiment of the invention a tertiary winding 20 comprising a number of turns of the order of 32 and a capacitor 2! having a capacity of the order of 0.075 microfarad provided for the coil ID a substantially flat impedance frequency characteristic over: approximately a 0.5-90 kilocycle range of frequencies. Thesimplicity of this embodiment was found to reside in the fact that an inexpensive and yet stable capacitor meeting relativel lenient minimum and maximum capacity requirements could be initially selected, and thereafter the number of turns of the tertiary winding 28 varied in order to obtain the neutralization of leakage inductance l 8 mentioned hereinbefore.

Fig. 2 shows the invention applied to a hybrid coil 25 comprising individual coils 26. and 21. The coil 26 wound on a magnetic core 23 embodies a winding 28 terminated in a suitable load 29 and shunted by a capacitor 38. Coupled to thewinding 28 are windings 31 and 32. Interposed between the two latter windings and the winding 28 is a shield 33. The coil 21 wound on a mag.- netic core 24 comprises a winding 35 terminated In accordance with the inventionas applied to Fig. 2, winding 50 adjustable as to the num-. ber of turns and embodied in the coil 26 and shunted by capacitor 5| selected similarly to capacitor 2| of Fig. 1 neutralizes substantially the portion of the leakage inductance 45 occurring in the coil 26 and remaining therein after the above-mentioned neutralizing action of the capacitor 3|]; and winding 52 adjustable as to the number of turns include-.2 in the coil 21 and bridged by capacitor 53 selected similarly to capacitor 2! of Fig. 1 neutralizes substantially the portion of the leakage inductance 46 occurring in the coil 21 and remaining therein after the previously pointed out neutralizing action of the capacitor 31. Consequently, the conjugate branches of the hybrid coil 25 are individually provided with substantially flat effective resistive and minimum reactive-frequenc characteristics that are substantially identical with those of the respective line 54 and loads 29, 36 and 43, when the remaining windings are properly terminated, looking ('1) into windings 3i and 38 from the carrier current line 54; or (2) into the winding 28 5 from the load 29; or (3) into the winding 35 from in a suitable load 36 and shunted by a capacitor 31. Coupled to the winding are windings 38 and 39. Disposed intermediate the latter two windings and the winding 35 is a shield 40. A link 4| serves to connect both shields 33 and 40 to ground 42. The windings 32 and 39 on the respective coils 26 and 21 are terminated in a load or balancing network 43. The windings 3| and 38 on the respective coils 26 and 21 are applied to terminals 44, 44 which are connected to a source 55 of alternating current waves and resistor 56 both of which simulate a carrier current line 54. This line together with the loads 29, 36 and 43 possess substantially flat effective resistive and minimum reactive-frequency characteristics for transmitting signaling waves extending'over a certain range of frequencies with substantially uniform attenuation.

A certain amount of leakage inductance occurs between the winding 28. and the windings 3| and 32, all of whichcomprise the coil 26, and may be represented for the purpose of this illustration as inductance 45 of Fig. 2. The capacitor 30 shunting the winding 28 serves t-'. neutralize substantially one-half of the total amount of the leakage inductance 45. Also, a certain amount of leakage inductance occurs between the winding 35 and the windings 38 and 39, all of which constitute the coil 21, and may be represented for the purpose of this illustration vas inductance 45 in Fig. 2. The capacitor 31 bridging the winding 38 serves to neutralize substantially one-half of the total amount of the'leakage inductance 46.

the load 36; and (4) into the windings 32 and 38 from the load 43.

Thus, the attenuation-frequency characteristics of the conjugate branches of the hybrid coil.

25 tend'to conform substantially with the corresponding attenuation-frequency characteristics of the carrier current line 54, and loads 29, 36 and 43, respectively. In other words, such neutralize tion of leakage inductance serves to balance the hybrid coil 25 like a bridge by providing similar impedances at the conjugate branches.

Accordingly, the neutralization of leakage inductance controls the reflection characteristic of the hybrid coil 25 so as to promote the operation of the entire carrier current system of Fig. 2 in the manner pointed out above in connection with Fig. 1. It is to be understood that windin 50 and capacitor 5| may also include the action of the capacitor 30 on the coil 26; and that the winding 52 and capacitor 53 may also include the action of the capacitor 31 on the coil 21, if desired. The advantage of splitting the capacity on each side of the leakage inductance is that the range of the hybrid coil is extended upwards in frequency.

Fig. 3 shows the invention applied to a hybrid coil comprising a winding 6| shunted by a capacitor 62 and terminated in a load 63. Conpled to the winding Si is a pair of windings 64 and 65 and load 66 applied in series across a pair of terminals 61, 61. Across one of the latter terminals and the midpoint of the windings 64 and 65 is connected a load 68. Across the terminals 61, 61 may be connected a source 51 of alternating current -v Wes and a resistor 58 both of which simulate a carrier current line This line together with the loads 63, 66 and B8 possesses substantially flat effective resistive and minimum reactive frequency characteristics for transmitting signaling waves extending over a shunting the winding 63 tends to neutralize substantially one-half of the total amount of the leakage inductance I I.

I respect to the load impedance.

The several windings are wound' on a magnetic core 15.

In accordance with the invention as applied to Fig. 3, vvinding 12 adjustable as to the number of turns and shunted by'capacitor 13 selected similarly to capacitor 2| neutralizes the portion of the leakage H occurring in the hybrid coil 60 and remaining therein after the aforedescribed neutralizing eifectt-of the capacitor 62. Accordingly, the conjugate branches of the hybrid coil 60 are individually provided with substantiallyfiat ef- Thus, the attenuation-frequency 'characteristics of the conjugate branches of the hybrid coil 60 tend to conform substantially with the cor responding attenuation-frequency characteristics of the carrier current line 51 and the loads 63, 66

and 68, respectively. This balances the hybrid coil 60 like a bridge as similar impedances' are provided at conjugate branches. Consequently, the'neutralization of leakage inductance controls the reflection characteristics of the hybrid coil 60 so as to promote the operation of the entire carrier current system of Fig. 3 in the manner discussed above regarding Fig. 1. It is to be understood that the winding 12 and capacitor 13 may also include the previously mentioned neutralizing action of the capacitor 62, if desired. Although the illustrations of Figs 1, 2 and 3 show magnetic core coils, it is to be understood that the invention is not necessarily limited to such use as it will achieve equally satisfactory re- 7,

identical with the attenuation-frequency characteristic of said one load circuit to which said one pair of winding termina is connected.

2. In the combination n broad band inductance apparatus according to claim 1, in which said impedance network comprises at least one further inductance winding adjusted as to the number of turns and coupled to said plurality of windings, and a capacitor applied across said further winding.

3. A broad band inductanceapparatus, comprising a pair of coupled inductance windings between which leakage inductance occurs, and means comprising a tertiary inductance winding coupled to said pair of windingsand a capacitor in shunt of said tertiary winding to control the reflection characteristic of said pair of windings such that said leakage-inductance is substantially neutralized'to provide said pair of windings for signaling waves extending over a certain range of frequencies, looking into individual windings,

with a substantially flat effective attenuation-frequency characteristic that conforms substantially with a corresponding attenuation-frequency characteristic of individual networks to which individual windings of said pair of windings are connected and which transmit said signaling waves of said certain frequency range.

- 4. A broad band inductance apparatus, comprising two coupled inductance windings between which leakage inductance occurs, a tertiary inductance winding coupled to said two windings and a capacitor in'shunt of said tertiary winding, said tertiary winding adjusted as to the number of turns and said capacitor proportioned to neutralize substantially said leakage inductance to sults with coils embodying air cores; and furthermore, the invention may be utilized to accomplish similar results with such coils in the frequency ranges of both voice and radio signaling waves. I

In the above description, it has been assumed that the initial inductance is relatively high with This is true of most line repeating coils which must meet severe modulation requirements. However, in other coils in'which the mutual inductance has a more finite value, it maybe sometimes advantageous to tune such inductance with a capacitor. In the latter connection, it is understood that the invention hereinbefore described may also be expeditiously utilized for this purpose.

What is claimed is:

1. In combination, in broad band inductance apparatus, a plurality of coupled inductance windings between which leakage inductance occurs, and means comprising an impedance network coupled to said plurality of windings to neutralize substantially said leakage inductance to provide said plurality of windings for signaling waves extending over a certain range of frequencies, looking into one pair of winding terminals applied to one load circuit having a preselected attenuation-frequency characteristic for transmitting said signaling waves extending over said frequency range when the other individual pairs of winding terminals are applied to other individual load circuits having attenuation-frequency characteristics substantially identical with that of said one load circuit with an attenuationfrequency characteristicwhich is substantially provide said two' windings for signaling waves extending over a certain range of frequencies, looking into one of said two windings, with substantially fiat eifective resistive and minimum' windings is connected and which transmit said signaling waves of said certain frequency range.

5. A broad .bandhybrid apparatus, comprising a pair of individual electrical coils, each of said coils embodying a plurality of coupled inductance windings between which leakage inductance occurs, one winding of saidplurality of windings on each of said coils beingterminated in one impedance having a preselected attenuation-frequency characteristic to transmit signaling waves extending over a certain range of frequencies, circuit means to connect serially a second winding on each of said coils to another impedance having an attenuation-frequency characteristic substantially identical with that of said one impedance and further circuit means to connect serially a, third winding on each of said coils to a further impedance having an attenuation-frequency characteristic substantially identical with that of said 7' one impedance, and impedance means on each of said coils to substantially neu- I tralize aii' leakage inductance, comprising a furtherjgiductance winding coupled to said plurality I of coupled windings, and, a'capacitor across said coils to which windings individual impedances are connected with an attenuation-frequency characteristic that conforms substantially with the corresponding attenuation-frequency characteristic of the individual impedances to which said windings of both said coils are connected.

6. A broad band hybrid coil, comprising a plu-i rality of inductance windings between which leakage inductance occurs, a first winding having a termination in one impedance which possesses a preselected attenuation-frequency characteristic to transmit signaling waves extending over a certain range of frequencies, a second and a. third winding connected to three other discrete impedances, whose attenuation-frequency characteris tics are substantially identical with that of said one impedance, such that two of said other impedances are in series with both said second and third windings and a third of said other impedances is disposed in bridge of common points between said second and third windings and said two other impedances, said impedances constituting conjugate branches of said hybrid coil, and an impedance network to neutralize substantially said leakage inductance, comprising a further inductance winding coupled to said plurality of windings, and a capacitor across said further winding, said network proportioned to provide said hybrid coil for said signaling waves extending over said certain range of frequencies, looking into individual conjugate branches, with an control the reflection'characteristic of said apparatus 'by neutralizing said leakage inductance so that said pair of windings, looking into one of said pair of windings while the other winding thereof is terminated in one of said pair of circuits, with a substantially fiat effective attenuation-frequency characteristic that conforms substantially with the corresponding characteristic of the circuit to which said one winding is connected.

9. In a broad band signaling wave transmission system comprising a plurality of discrete circuits, each having a substantially flat effective attenuation-frequency characteristic for signaling waves extending over a certain, range of frequencies, a hybrid apparatus comprising a pair of individual electrical coils, each coil embodying a plurality of coupled windings between which leakage inductance exists, said hybrid apparatus being connected to said circuits to transmit therebetween said signaling waves of said certain frequency range such that a first winding of each coil is connected to one circuit, a second winding of each coil is connected to another circuit, and a third winding of each coil is connected to a further circuit, and means comprising a further inductance winding adjustable as to the number of turns and coupled to said windings of each of said pair of coils and a capacitor in shunt of said further winding to control the reflection characteristic of said hybrid apparatus by neutralizing substantially said leakage inductance so that said sion system including a plurality of discrete circuits, each having a substantially flat effective attenuation-frequency characteristic for signaling waves extending over a certain range of frequencies, inductance apparatus comprising at least one plurality of coupled inductance windings connecting operatively discrete circuits of said plurality of circuits with each other to transmit therebetween said signaling waves of said certain frequency range and having an indeterminate amount of leakage inductance between said coupled windings, and means comprising an inductance winding adjustable as to the number of turns and coupled to each said plurality of windings and a capacitor applied across each said adjustable winding to control the reflection characteristic of said plurality of windings by-neutralizing substantially said leakage inductance so that said plurality of windings, looking in one winding of said plurality of windings while the other windings thereof are applied to other circuits of said plurality of discrete circuits, possesses a substantially flat effective attenuation-frequency characteristic that conforms substantially with the corresponding characteristic of said one circuit to which said one winding is connected.

8. In a broad band signaling wave transmission system including a pair of discrete circuits, each having a substantially flat efiective attenuationfrequency characteristic for signaling waves extending over a certain range of frequencies, inductance apparatus comprising a pair of coupled inductance windings between which leakage inductance occurs and connecting said pair of circuits such that individual windings are connected to individual circuits to transmit therebetween said signaling waves of said certain frequency range, and means comprising a further inductance winding adjustable as to the number of turns and coupled to said pair of windings and a capacitor in shunt of said further winding to hybrid apparatus, looking into winding terminals connected to one discrete circuit while the remaining winding terminals are applied toother discrete circuits, is provided with a substantially flat effective attenuation-frequency characteristic that conforms substantially with said corresponding characteristic of the one discrete circuit to which said looked-into winding terminals are connected.

10. In a broad band signaling wave transmission system comprising a plurality of discrete circuits, each having a substantially flat effective attenuation-frequency characteristic for signaling waves extending over a certain range of frequencies, a hybrid coil comprising a plurality of coupled windings between which leakage inductance exists and connecting said plurality of circuits to transmit therebetween said signaling waves of said certain frequency such that one circuit is connected to a first winding, two other circuits are serially connected to second and third windings disposed in series, and a further circuit is connected in bridge of common points of said two other circuits and said second and third windings, said circuits constituting conjugate branches of said hybrid coil, and means comprising a further inductance winding adjustable as to the number of turns and coupled to said windings and a capacitor in shunt of said further winding to control the reflection characteristic of said hybrid coil by neutralizing substantially said leakage inductance to provide said hybrid coil, looking into the winding terminals connected to one discrete circuit while the remaining winding terminals are applied to other discrete circuits, with a substantially flat eflective attenuation-frequency characteristic that "conforms substantially with said corresponding characteristic of the one discrete circuit to which said looked-into winding terminals are connected.

NELSON BOTSFORD. 

